Missing inductance on transformer?

[szzuk]

I built a transformer for my circuit and I don't think it is so good, I was wondering if someone could confirm what is happening?

The circuit is a piezo that operates at resonance. 2V comes from an amplifier into the transformer, the piezo and inductance L3 are matched for resonance so I have around 32V across the piezo. The circuit rings/works but I have a few questions.

1) I built the transformer to have inductance of 15 and 400uH. In theory this should be a turns ratio of 1:5 with an inductance ratio of 1:25. But if I test the transformer it appears to be around 1 : 6.5, so I have 'lost' inductance and as I wound measuring inductance I have done something wrong. How important is wiring in an orderly fashion? How do I build a 'good' transformer by winding?

2) To set up resonance I have to match the piezo capacitance with L3. At the right frequency, 12KHz with a piezo cap of 420nF, in theory this is around 400uH. So L3 should be 400uH, incidentally the same as L2. But I have tested the circuit and it works better with L3 at 570uH. What is happening? Does the 'lost' inductance somehow have to be recovered for the best resonance to occur?

Szzuk.

 

[szzuk]

I've been reading up on transformer winding best practise. It seems to be:

a) Interleaving. What is that exactly?
b) Leave no spaces between windings
c) I have a gapped core so put the windings across the gap
d) Air spaces hold capacitance perhaps this is upping the L3 inductance

Is there anything else?

Thanks,

Szzuk.

 

[Diver300]

The inductance of the transformer secondary is massively changed due to the impedance of the source that is driving the primary.

Interleaving is winding some of the primary, then some of the secondary, then some of the primary, then some of the secondary, and so on. For transformers with a large turns ratio, the simplest interleave is to wind half the larger number of turns, then wind the smaller number, then the rest of the larger number of turns.

 

[szzuk]

I have done some impedance calcs from direct measurement, the primary impedance looks to be around 0.7 ohms and the secondary around 25 ohms. It is difficult to know the piezo impedance at resonance because there is no datasheet, but it is probably <5ohms. The power source is an opamp, the opa548.

Thanks for the heads up with interleaving. I googled to find out what it is but the explanations were way to complicated !